Rleff, the key to that is a flat impedance curve. The Avalon Eidelon does that well, as well as most magnetic planars, whose impedance curve has nothing to do with box resonance :)
There is another issue- the difference in performance between low impedance speakers and high impedance. Actually, the difference in impedance has little to do with the speakers and everything to do with the amps: **all amplifiers** regardless of the technology, will sound better driving higher impedances. IOW if sound quality is your goal, it will be better served by an 8 or 16 ohm speaker as opposed to 4 ohms, all other things being equal.
Another way to put this is if you had a speaker with 2 woofers, 2 midrange units and 2 tweeters, and all the drivers were 8 ohms, it would sound better wired as a 16 ohm load than it would wired as a 4 ohm load, regardless of the type of amplifier involved.
OTOH if sound **pressure** is your goal, and you have a transistor amplifier, a 4-ohm speaker might be preferred.